Final answer:
A nonfunctional lacI mutant can be rescued by a wild-type lacI allele because the lacI gene product acts in trans and thus, can diffuse within the cell to control any copy of the lac operon.
Step-by-step explanation:
The result obtained in Jacob and Monod's merodiploid analysis suggested that the lacI region acts in trans to regulate the lac operon. This means that a nonfunctional (lacI- | lacO | lacY | lacZ | lacA) mutant can be rescued by a wild-type (lacI+ | lacO | lacY | lacZ | lacA) allele. The lacI gene produces a repressor protein that can diffuse through the cell and act on any copy of the lac operon. If the lacI gene is mutated and nonfunctional, a separate copy of a functional lacI gene on another part of the DNA or on an F' episome can still produce the repressor protein that controls the operon, thereby 'rescuing' the regulation of lac operon.
It is important to understand that this is a case of complementation, where the product of the wild-type lacI gene can compensate for the loss of the same gene product from the mutant allele. The lac operon consists of a promoter, an operator and the structural genes lacZ, lacY, and lacA. The operator is the site where the repressor protein binds and regulates the transcription of the structural genes.